The Spirochaetes are one of the eleven eubacterial phyla. They are easily recognisable by unusual morphological characteristics such as their helical shape and the presence of periplasmic flagella. In recent years, in depth studies of spirochaetal genomes have revealed unexpected characteristics including linear chromosomes and plasmids for Borrelia, two circular chromosomes for Leptospira and unusual ribosomal gene organisations in several spirochaetes. For Leptospira, physical and genetic mapping of the large chromosome suggested that the enormous diversity between serovars is most likely due to large internal rearrangements. Similar mapping studies, and sequencing of genes from different species of Borrelia revealed a strictly clonal mode of evolution for this organism's chromosome. E. coli can no longer be considered as the bacterial paradigm and dogma such as the unicity and circularity of the bacterial chromosome and the organisation of rRNA genes in operons do not apply to the Spirochaetes which are clearly a unique group in the bacterial world.